The purpose of this Research Core is to foster collaboration and sharing of data and methods among the members of the group in a manner which will enhance existing Center projects and motivate proposals for future studies. The specific aims of the Toxicokinetic Susceptibility Research Core are to: 1) support and expand collaborative research in toxicokinetic and toxicodynamic susceptibility that builds on both stochastic and mechanistic traditions; 2) establish effective mechanisms for sharing ideals, discussing research in progress, and fostering new interdisciplinary ventures through regular Core meetings; 3) promote innovative research that incorporates population approaches into biomarker studies; and, 4) foster state-of-the-art biomarker measurements into research projects of other Center investigators, particularly field epidemiologists.